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78

IRON MORDANTS

and of any impurities (especially iron), which are detected by the ordinary
methods, and especially determination of the chromium, for which the
two following methods serve :

(a) GRAVIMETRIC. The substance (5-10 grams of a solution or 1-2
grams of a solid) is diluted with or dissolved in water (the hydroxide in
hydrochloric acid), excess of ammonium chloride or nitrate and slight
excess of ammonia being added and the solution boiled until the liquid
above the precipitate is quite decolorised. After filtration, the precipitate
is washed witli water containing ammonium nitrate, dried, ignited and
weighed as Ci 203; i part of Cra03 =0-6853 part Cr.

(6) VOLUMETRIC. 10 grains of solution (or I gram of solid) are dis-
solved in water or hydrochloric acid and the volume made up to 100 c.c. ;
10 c.c. (  i gram of original solution or o-i gram of a solid salt) are. treated
in a litre porcelain dish with concentrated sodium hydroxide solution until
the precipitate initially formed reclissolvcs. The dish is then heated on a
steam-bath and sodium peroxide added in small amounts until a perfectly
yellow solution is obtained (the chromium being transformed into chro-
mate),.1 The liquid is then evaporated to dryness, the residue taken up
in water, and the dichromate in the solution estimated iodome.trically as
described under Potassium Didiromate (5, h) : i c.c. of N/io~thiosulphate
 0-002533 gi'ain of CrB0;i.

If the basicity of the chromium salt is desired, the acid must be deter-
mined and the amount of acid corresponding with i part of chromium (see
Aluminium Sulphate, 3) calculated.

IRON  MORDANTS

These include the sulphate find acetate (see separate articles) and solu-
tions of Haste (erne sulphate and Ferric 'nitrate, which are known as iron
mordant or /ei'i'it^ine, These solutions are reddish-brown liquids of D i*35~-
1-56, corresponding with 40-52" Baum.e', and they contain, besides ferric
sulphate or nitrate or both of these salts, also ferrous and alkali salts. These,
products are analysed as follows :

1.  Nature of the Mordant. -The dilute solution is treated with hydro-
chloric add and barium chloride ; if sulphate is present an abundant white
precipitate of barium sulphate is formed.   A second portion of the solution
is evaporated to dryness on the water-bath and the residue treated with a
few pieces of copper and cone, sulphuric, add : if nitrate is present, evolu-
tion of red vapours occurs.

2.  Iron.10 grams of the substance arc diluted to 100 c.c. with water
and 10 c.c. of this solution (= i gram of substance) boiled with a few drops
of nitric acid the iron being then precipitated with ammonia and weighed
as ferric oxide, this gives the total iron (i part Fea03   0-7 part Fe).
Another aliquot part of the dilute solution is acidified with pure, sulphuric

1 If the product to be analysed contains organic salts or substances, it should be
fused with solid sodium hydroxide (1-2 grams) in a nickel crucible and a small quantity
of sodium peroxide then added. After cooling, the. mass is dissolved in water and the
dichromate in the solution determined iodometrically. XV, z, p, 32j, and Hull, chim. /arm,,